In a conventional IBM mainframe environment a central processor unit (CPU) has typically a plurality of input/output channels which are connected by cables to communicate in an interlocked manner with peripheral devices through devices known as control units. In a typical connection an IBM channel will have input and output bus and tag lines, special interlock and control lines, each of which is a coaxial cable that is carefully shielded and terminated to maintain the integrity of the signal lines. These lines are required by IBM to be limited in length in order to preserve certain timing constraints in an interlocked communication operation between a CPU channel and a control unit. Hence, even if the initial expense of larger coaxial cables is warranted to, for example, place peripheral devices in another building than where the CPU is located, the timing constraints tend to limit the separation distance.
The communication protocol between an IBM mainframe CPU channel and external devices through a so-called I/O interface has been publicly described. One such publication is by IBM itself and is entitled, IBM System/360 and 370 I/O Interface Channel to Control Unit OEMI, published originally in 1971 under GA22-6974. Patents on the I/O interface have issued, such as U.S. Pat. Nos. 3,336,582, 3,400,372 and 3,582,906 to Beausoleil et al and many others. A key feature of the channel I/O interface is that the rise and fall of all signals transmitted over the interface are generally controlled by interlocked responses.
Techniques have been proposed to extend the IBM channel so that it can communicate at high speeds with a remotely located control unit. One such technique proposed by IBM itself is known as the 3044 system and enables a channel to communicate with a remotely located control unit through a fiber optic data communication link. The product, as described in IBM's Publication Number GA22-7097, extends the channel-to-control-unit distance to a maximum of 2000 meters (6,600 ft). The effect of the extender as a practical matter reduces the effective data rate, thus undesirably increasing the channel busy time. This speed reduction is believed to arise by virtue of the interlocked operation, by which for each data byte that is transmitted at least two or four additional link trips are needed to complete a transfer of the data byte.
When CPU's at remotely located sites are desired to be connected, a well known technique involves a teleprocessing link and a pair of so-called front-end-processors (FEP). Such FEP to FEP link, however, downgrades the speed at which the CPU can communicate with a control unit and thus limits the information flow.
Another system proposes a remote communication between an IBM CPU and a control unit. This system has a remote control unit that interfaces with remotely located peripheral devices and has a local control unit that interfaces with a CPU channel. Data from a channel is formatted into standard frames suitable for transmission through a network. The operating speed between the CPU and a remote control unit is limited in a full duplex mode to about 56 kilobits per second which is far below channel speeds that may be as high as 3 megabytes per second.